MNG decoder performance very bad

Hello List,

I'm trying to use LibMNG (with ZLib and LCMS) to play back animated
clips of medical data (no videos of persons, but animated technical
schematics). MNG seems perfect for the job, because the animations have
no sound, and also the compression should be lossless (no artifacts).
The MNG files have been converted from uncompressed AVIs using JASC
Animation Shop, then fix them using the utility on the web site (I tried
some other tools, but found none which was able to both import AVI and
write a MNG file and didn't crash or produced strange error messages.
Maybe someone has an idea).

My problem is that decoding is very slow (it doesn't reach the frame
rate I need, though I have a P-IV 2,4 GHz and 512 MB RAM) and it's
allocating LOTS of memory.

I have tried a lot of things, e.g. storing chunks is off, I preload the
entire MNG into memory to avoid buffering issues etc. But to no avail.

Let me give some figures. For example, one file is ~4 MB big and
contains 31 frames of size 765x486. Framerate is 10 fps. Even though I'm
preloading the whole image into memory, LibMNG can only play this file
at about 5 fps. Also, everytime mng_display_resume() is called, there is
~170 kB memory allocated. After all 31 frames have been shown, the
animation loops and then plays the 10 fps without problems. So it seems
to me that the library is storing all frames in memory, and the
decompression takes so much time.

I have checked with the Visual C++ profiler to ensure that it isn't some
glitch inside my own drawing code, but this is the result:

Time (ms)  Function
----------------------------------------------
1401,371  _filter_paeth (libmng_filter.obj)
1283,284  MngAlloc(unsigned int) (manimationwindow.obj)
706,850   mng_display_bgr8 (libmng_pixels.obj)
403,552   _inflate_fast (inffast.obj)
207,538   _adler32 (adler32.obj)
192,358   _mng_store_rgba8 (libmng_pixels.obj)

This means that more than 4 seconds are needed just for the internal
decoding functions of the video which should play in approx. 3 seconds.

So did I miss something or is this task just too difficult for MNG?

Best regards


Markus Meyer



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